The number of patients suffering from glaucoma has increased sharply in recent years. Most patients who become aware of glaucomatous symptoms and see an ophthalmologist are found to be in the final stage of glaucoma and, in most cases, 80% or more of the optic nerve fibers of those patients have exfoliated.
The eyeball is filled with vitreous body and aqueous humor, and the shape of the eyeball is maintained by the pressure of the aqueous humor, i.e., by intraocular tension. The intraocular tension must be in a physiologically appropriate range. If the intraocular tension is excessively high, the circulation of the aqueous humor is obstructed or the optic nerves are under pressure and are caused to atrophy. Glaucoma is a functional illness resulting from excessively high pressure within the eyeball which entails a considerably high probability of loss of sight.
The prevention of optic nerve impairment and the suppression of progress of optic nerve impairment are important therapeutic objectives in the treatment of glaucoma. The diagnosis of the symptoms of glaucoma in the earliest possible stage of functional impairment and appropriate management and therapy are essential to achieve such objectives.
An abnormal intraocular tension and abnormalities in the retina and in the visual field and the like are typical symptoms of glaucoma. Symptoms are diagnosed on the basis of information obtained by integrating measurements of intraocular tension, examination of the retina, and tests of visual functions including the visual field to identify glaucoma in the early stages of progress.
To improve the accuracy of examination, a known glaucoma analyzer determines parameters including the C/D ratio, i.e., the ratio of optic cup diameter to optic disc diameter, indicating the size of the optic disc recess, the average recess in the optic cup, and the vertical, transverse and diagonal widths of the rim on the basis of the results of three-dimensional measurement of the optic disc on the retina. The parameters are applied to a neural network to estimate defects in the visual field and the degree of progress of glaucoma.
To accurately know the defects of nerve fiber bundles, it is necessary to conduct full mydriasis, irradiate the retina brightly, accurately focus on the nerve fiber bundles, strengthen the reflected information from the nerve fiber bundles, and make a contrast with the information.
Conventionally, glaucoma patients are distinguished from healthy people by taking a red-free retina photograph, calculating the gradation density of the nerve fiber bundles on the vertical line near the center of the macula lutea-optic disc line or around the optic disc by using computer processing or the like, and comparing these data with data from healthy people.
Since the simple C/D ratio is not a sufficiently effective parameter, another glaucoma analyzer relies on the fact that the nerve fiber bundles occupy specific areas on the optic disc. This type of analyzer sets a fixed optic disc quantity pattern, compares the image of the optic disc on the retina obtained by direct observation or by photograph with the fixed optic disc quantity pattern, and diagnoses the degree of progress of glaucoma on the basis of the comparison and information about defects in the nerve fiber bundles.
The optic disc is divided into twelve sectors according to the arrangement of the optic nerve fibers. A red-free retinal photograph is read by a digitizer. The area of the optic disc and that of the optic disc recess are determined. The C/D-A ratio (the ratio of the area of optic disc cupping) is calculated. The obtained information is examined to estimate the degree of progress of glaucoma.
Since these known glaucoma analyzers analyze the optic disc or the nerve fiber bundles individually or locally, they are incapable of achieving comprehensive analysis and diagnosis of glaucoma in comparison to doctors, who analyze and diagnose glaucoma on the basis of integrated information about the optic disc, the retina and the visual field.
Since these known glaucoma analyzers are not provided with any computing system for automatically processing all the data including those of the optic disc, the retina and the visual field, these known glaucoma analyzers must carry out a troublesome operation to read the red-free retinal photograph by the digitizer. Such an analyzing procedure is quite inefficient, and the reproducibility of data acquired by this analyzing procedure is not satisfactory.
The nerve fiber bundle defect on the retina is one of the conditions symptomatic of glaucoma. Since the reflectance of the defective portion of the retina in which defects are found in the nerve fiber bundles is lower than that of the normal portion of the retina, the defective portion appears in a wedge-shaped dark streak on a retinal photograph. Nerve fiber bundle defects extend outward from the optic disc in curves along the arrangement of optic nerves. Thus, the optic nerve fiber bundle defects extend radially from the optic disc and curve sharply near the macula lutea.
A first known method of detecting defects in optic nerve fibers involves scanning a red-free retinal photograph to determine the density level along a fixed straight line or along radial lines extending from the optic disc. When the density of a specific area within the scanned area is lower than an expected density, a portion of the retina corresponding to this specific area is regarded as a defective portion.
More specifically, a picture is scanned linearly to detect defects in the optic nerve fibers through the measurement of the density of the picture along a straight scanning line which is the perpendicular bisector of a line segment between the optic disc and the fovea centralis. A density distribution in an angular range of 30.degree. corresponding to a visual field about the intersection of the line segment and the perpendicular bisector is analyzed, and the results of the analysis are compared with corresponding data for an age bracket in which the subject is included.
A second known method of detecting defects in nerve fiber bundles involves scanning an area around the optic disc in a retinal image along radial scanning lines extending in different directions from the optic disc. A direction in which defects in the nerve fiber bundles exist is determined on the basis of cumulative densities respectively on the radial scanning lines. The cumulative density data obtained by scanning is plotted in a linear graph in which directions from the optic disc are measured on the abscissa axis and the cumulative densities are measured on the ordinate axis. Then, the linear graph is searched for a minimal point, and it is decided that lesions exist in the direction corresponding to the minimal point. Since it is possible that blood vessels are mistaken for lesions, the prior discrimination of the directions in which blood vessels are expected to exist from other directions are made beforehand to prevent the misidentification of hollows in the graph corresponding to the blood vessels and lesions.
Since the first known method, which involves scanning the retinal image along a linear scanning line to detect defects in the optic nerve fibers, analyzes the retinal image locally for defects in the nerve fiber bundles, lesions existing in areas other than the scanned area cannot be found thereby. Since the second known method, which involves scanning the retinal image along radial lines radiating from the optic disc, scans only an area around the optic disc, the divergence between the shapes of defects in the nerve fiber bundles and the scanned area increases if the scanning area is expanded, resulting in reduced measuring accuracy.
When measuring the visual field, importance is attached to the Bjerrum area and the nasal step rather than to an area around the optic disc. Data representing direction and distance from the optic disc is effective in specifying a portion corresponding to the Bjerrum area and the nasal step on the retina. Therefore, there has been a strong demand for a retinal disease analyzer capable of acquiring such data and of accurately specifying an area on the retina in which loss of visual field is likely to occur, taking the shapes of optic nerves into consideration. It has also been desired to make a comprehensive determination of the direction of expansion of the pallor, the curvature of the blood vessel around the edge of the optic disc and defects in the nerve fiber bundles, which appear along the optic nerve fiber distribution pattern.
Typically, ophthalmologists have determined the positions and sizes of defects in the retina quantitatively by rough estimation.
A further known method of detecting defects in nerve fiber bundles involves measuring the optic disc by a manual measuring operation and determining the position of the optic disc by an automatic measuring operation. A point having maximum values for r(.lambda.), g(.lambda.) and b(.lambda.) is within a portion of a retinal image corresponding to the optic disc. Therefore, in the automatic measurement of the optic disc, a specific point having maximum values for r(.lambda.), g(.lambda.) and b(.lambda.) is detected. Values of points above, below, on the right side and on the left side of the specific point for r(.lambda.), g(.lambda.) and b(.lambda.) are compared with the maximum values of the specific point, and a point at which the difference between the maximum values of the specific point and the values of the point exceeds a threshold is regarded as a point corresponding to the optic disc.
This known retinal disease analyzing method is incapable of automatically detecting the optic disc and the macula lutea in the retinal image and of automatically determining coordinates. Manual determination of coordinates is quite inefficient and unreliable.
Accordingly, there has been a strong demand for a retinal disease analyzer capable of precise determination of the arrangement of the optic nerve fibers and of using frequencies of diseases to determine regions in the eye.